This solves an issue with installation showing "undefined" alert messages reported in #5331. Issue was introduced by me in #5257

Description

Basically the installation process is not adding alert message type translations properly (Before using any message type you have to add the JS translation with JText::script('MESSAGE'); , JText::script('ERROR');, etc.). If not the message is not really translated. This is discovered after removing the jQuery dependency in #5257

Basically what I'm doing now is testing that the translation of the message type has been defined and if not we skip adding the alert heading. This preserves exactly the previous behavior. Of course it would be better to add that message types translations to the installer but I think this fix makes sense anyway if third part extension developers forget to add the translation for example.

Backward compatibility

This PR has no B/C issues. In fact it fixes one.

Test

On the first Joomla installation screen of Joomla do not fill any fields and press the Next button. You should see the undefined alert message titles. After applyting the patch no titles should be visible.